Test identity multiplication

rtchallenge/src/matrices.rs

@@ -57,6 +57,21 @@ impl From<[f32; 16]> for Matrix4x4 {
 
 impl Matrix4x4 {
     /// Create a `Matrix4x4` containing the identity, all zeros with ones along the diagonal.
+    /// # Examples
+    ///
+    /// ```
+    /// use rtchallenge::matrices::Matrix4x4;
+    ///
+    /// let a = Matrix4x4::new(
+    ///     [0., 1., 2., 3.],
+    ///     [1., 2., 4., 8.],
+    ///     [2., 4., 8., 16.],
+    ///     [4., 8., 16., 32.],
+    /// );
+    /// let i = Matrix4x4::identity();
+    ///
+    /// assert_eq!(a * i, a);
+    /// ```
     pub fn identity() -> Matrix4x4 {
         Matrix4x4::new(
             [1., 0., 0., 0.],